Cosmic microwave background, matter-antimatter asymmetry and neutrino masses

We study the implications of thermal leptogenesis for neutrino parameters. Assuming that decays of N-1, the lightest of the heavy Majorana neutrinos, initiate baryogenesis, we show that the final baryon asymmetry is determined by only four parameters: the CP asymmetry epsilon(1), the heavy neutrino mass M-1, the effective light neutrino mass (m) over tilde (1), and the quadratic mean (m) over bar of the light neutrino masses. Imposing the CMB measurement of the baryon asymmetry as constraint on the neutrino parameters, we show, in a model independent way, that quasi-degenerate neutrinos are incompatible with thermal leptogenesis. For maximal CP asymmetry epsilon(1), and neutrino masses in the range from (Deltam(sol)(2))(1/2) to (Deltam(atm)(2))(1/2) the baryogenesis temperature is T-B = O(10(10)) GeV. (C) 2002 Elsevier Science B.V. All rights reserved.